Microfibrillated cellulose (MFC) in accordance with the present invention relates to cellulose fibers that have been subjected to a specific mechanical treatment in order to increase the specific surface and to reduce the size of cellulose fibers in terms of cross-section (diameter) and/or length, wherein said size reduction preferably leads to a fibril diameter in the nanometer range and a fibril length in the micrometer range.
Microfibrillated cellulose (also known as “reticulated” cellulose or as “superfine” cellulose, or as “cellulose nanofibrils”, among others) is described, for example, in U.S. Pat. Nos. 4,481,077, 4,374,702 and 4,341,807. According to U.S. Pat. No. 4,374,702 (“Turbak”), microfibrillated cellulose has properties distinguishing the same from previously known cellulose products.
MFC in accordance with the “Turbak” process (see FIG. 5, Technology B) is produced by passing a liquid suspension of cellulose through a small diameter orifice in which the suspension is subjected to a pressure differential and high velocity shearing impact, followed by a high velocity decelerating impact, until the cellulose suspension becomes substantially stable. This process converts the cellulose into microfibrillated cellulose without inducing substantial chemical change to the cellulose as such. The microfibrillated cellulose is present in the form of individual fibrils or as fibril bundles (fibrils arranged together in a bundle).
An improved process for obtaining particularly homogeneous MFC is described in WO 2007/091942.
A representative depiction of conventional MFC as known from the art is shown in an optical microscopy photograph in FIG. 1. The corresponding process known from the art as used to obtain this conventional MFC is shown schematically in FIG. 5 (Technology C).
Due to the large surface area and high aspect ratio (ratio of fibril length to fibril width), microfibrillated cellulose generally has a good ability to form stable three-dimensional networks, in solution (including water and organic solvents). In solution, MFC typically forms a highly viscous gel-like dispersion with shear thinning properties. This means, among others, that microfibrillated cellulose has a good ability to stabilize dispersions in a stationary state, while, at the same time, rendering the dispersion easier to process, for example in regard to pumping. This is because the viscosity of the MFC dispersion is reduced when shear forces are applied. As a consequence, zero shear viscosity, η0, is comparatively high, while the viscosity at high shear is comparatively reduced.
However, the MFC known from the art, while showing shear thinning properties, has limitations in regard to water retention (capability to retain water) and/or in regard to the degree of zero shear viscosity, η0, that can be reached in certain solvents, for example in polyethylene glycol (PEG). This deficiency becomes apparent, in particular, in high performance applications of MFC in coatings, paints, adhesives, cosmetics, home care products, pharmaceuticals (ointments) etc.
It is therefore an object of the present invention to provide microfibrillated cellulose, which shows improved rheological properties in dispersion in a solvent, in particular in polyethylene glycol (PEG), and/or microfibrillated cellulose which has improved water retention properties, in dispersion, among others.